Photovoltaic generation has been highly expected as an energy source alternative to thermal power generation and atomic power generation. The production volume of solar cells in recent years has increased dramatically. For solar cells, crystalline solar cells, solar cells formed by using a single crystal or polycrystalline silicon substrate, or thin-film solar cells, solar cells formed by depositing a silicon thin film on a glass substrate, are used. A unit of installation of solar cells in a photovoltaic power system is a solar cell module. A plurality of solar cells described above is connected in series or in parallel in a panel according to a purpose, and is provided with a frame constituting an outer frame and terminal boxes, to function as a solar cell module. A large number of solar cell modules mounted on a base, a power transmission cable, a power conditioner, and others are combined to constitute a photovoltaic power system. Such systems are not limited to general home power generation applications, and are also used in large-scale photovoltaic power plants having an amount of power generation of 1 MW or higher.
Solar cell modules do not have mechanically operating portions, and their lives are said to be generally twenty years or longer. However, in actuality, there have been reported cases where malfunctions occurred within several years or less after the start of operation due to various causes. Known causes of malfunctions include increased resistance due to degradation of a power generation layer in solar cells or corrosion of electrode portions, reduced optical transmittance of a sealing material that fills surrounding areas of solar cells to protect the solar cells, degraded insulation, increased wiring resistance in a solar cell module, and grounding failure of a metal base to which a solar cell module is fixed. These malfunctions cause reduced output of a solar cell module, and may finally lead to faulty functioning. A diagnostic technique capable of detecting such degradation states of solar cell modules at an early stage has been required to increase the reliability of photovoltaic power systems and to spread the use of them further.
When one or some solar cell modules in a photovoltaic power system fail, a malfunction can occur over the entire system. Therefore, it is ideal to periodically determine whether degradation is occurring on each solar cell module, and to repair or replace solar cell modules with appropriate timing. This requires a degradation diagnosing technique or a failure predicting technique for solar cell modules.
As matters now stand, as a method for checking the operating condition of a solar cell module, a method for measuring generated current or voltage and monitoring the amount of power generation is typical. However, the amount of power generation of a solar cell module varies greatly depending on an external factor such as the amount of solar radiation or weather conditions at the time of measurement. Therefore, only by monitoring the current, the voltage, or the amount of power generation of a solar cell module, it is difficult to determine whether the module is operating normally. Specifically, by monitoring the power generation amount as described above, the so-called “0” or “1” determination such as “operating” or “hot operating” is possible, but it is difficult to determine whether an abnormality has occurred in a solar cell module based on a situation where the amount of power generation has decreased in an actual installation environment where the amount of solar radiation varies every moment. Further, when a photovoltaic power system is constructed, it is difficult to determine on site whether there is a malfunction in connections between modules or there is a problem in modules themselves at the time of completion of the construction.
To this situation, a module diagnostic method using high frequencies has been proposed in recent years. Ac voltage of various frequencies is applied to a solar cell module using a variable-frequency signal generator to measure the frequency dependence of the impedance of the solar cell module, based on a current and voltage waveform at each frequency. From a frequency characteristic curve obtained by the measurement and impedance frequency response characteristics given by an equivalent circuit model of the solar cell module, equivalent circuit constants, characteristic variables unique to the module measured, can be obtained. By comparison between the values of these constants and values when the panel is normal, an increase in series resistance of electrodes or wiring can be detected to detect the occurrence of excessive contact resistance. As an example of the equivalent circuit model for the solar cell module, an equivalent circuit including four circuit elements, inductance L of tab wiring and output cables of the module, series resistance Rs of wiring and electrode portions, and junction capacitance Cd and insulation resistance Rsh of a power generation layer of the solar cells is used (see Patent Literature 1, for example).